In an image forming system, an optional sheet post-process apparatus can be connected to an image forming apparatus such as a multifunction peripheral. Recently, a sheet post-process apparatus is proposed which has a function that aligns ends of a stack of sheets printed by the multifunction peripheral are aligned in length (longitudinal) and width (lateral) directions, and performs saddle stitch binding of the stack of sheets to obtain a booklet.
As the sheet post-process apparatus, US Patent Application Publication No. 2004/0254054A1 discloses a sheet folding device that pushes out a folding plate in the direction perpendicular to a vertical sheet conveying path to insert the sheet or sheet stack between a pair of folding rollers and fold the sheet or sheet stack nipped and fed by the folding rollers. In the paper folding device, the rear ends (lower ends) of the sheets stacked between stack conveying guide plates present along a sheet conveying path are supported by a movable rear end fence and elevated along the sheet conveying path.
Conventionally, there is a known structure for stitch binding. In the structure, a stapling unit is disposed on an upper side of a pair of folding rollers along a sheet conveying path, and a pair of lateral alignment plates are disposed on an upper side of the stapling unit along the sheet conveying path. The lateral alignment plates are exposed in the same plane as a stack conveying guide, and include a pair of jogger fences which align the both side ends of the sheet stack. When the longitudinal center portion of the sheet stack is set to the processing position of the stapling unit, these lateral alignment plates perform a lateral aligning operation of moving in a width direction of the sheet stack so that both side ends of the sheet stack are temporarily aligned with the jogger fences.
In an example where the stapling unit is of a separated type including a driver unit and an anvil unit which are opposed on the both sides of the sheet conveying path, a sheet feeding guide is disposed between the folding unit and the stapling unit. The anvil unit causes the staple surface of the driver unit to sink together with the sheet stack by about 10 mm at the time of stapling. In view of this, the sheet feeding guide is disposed at a position lower than the staple surface by about 10 mm and has a portion which extends to the staple surface of the driver unit and is capable of being depressed.
If a step is present between the sheet feeding guide and the stack conveying guide located on a lower side of the folding unit, distortion of the sheet stack cannot be symmetric between the upper and lower sides. This raises a problem that the folding plate folds the sheet stack at a portion deviated from the longitudinal center portion of the sheet stack. That is, it is difficult to ensure folding position accuracy.
Further, the stack conveying guide has a bead structure that improves smooth movement of sheets. However, if the bead structure is applied to exposed surfaces of the lateral alignment plates, this raises a problem that the sheets tend to be caught on beads during the lateral alignment operation. To cope with this problem, the bead structure may not be applied to the lateral alignment plates. However, this raises another problem that the sheet stack is not conveyed from the processing position of the stapling unit to a folding position. That is, if static electricity applied during an electrophotographic printing process remains in the sheets, the sheet stack is adhered to the exposed surfaces of the lateral alignment plates due to the static electricity. The adhesion force is large enough to prevent the sheet stack from sliding down by its own weight when the position of the movable rear end fence is changed.